Stereom is a calcium carbonate material that makes up the internal skeletons found in sea urchins, and all other echinoderms, both living and fossilized forms. It is a sponge-like porous structure which, in a sea urchin may be 50% by volume living cells, and the rest being a matrix of calcite crystals. The size of openings in stereom varies in different species and in different places within the same organism. [1] When an echinoderm becomes a fossil, microscopic examination is used to reveal the structure and such examination is often an important tool to classify the fossil as an echinoderm or related creature. [2]
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